Method of removing oxide coatings from metals



Patented May 24, 1932 UNITED STATES PATENT OFFICE HARRY S. GEORGE, OFMASSAPEQUA, NEW YORK, ASSIGNOR TO ELECTED METALLURGICAL COMPANY, A.CORPORATION OF WEST VIRGINIA METHOD OF REMOVING OXIDE COATINGS FROMMETALS Ho Drawing. Application filed December 21, 1928. Serial No.827,756.

My invention relates to methods of removing oxide coatings frommetalarticles and especially to methods of removing such oxides withoutattacking the metal.

The method consists broadly of treating certain conditions of surfaceoxides in an alkaline bath and then removing the oxide or scale in anacid bath. The alkaline bath may be a solution of an alkali, such assodium, potassium or calcium hydroxide and it preferably contains anoxidizing agent, such as chlorine or its oxygenated derivatives, sodiumhypochlorite, hydrogen peroxide, or bromine. A satisfactory treatingbath may consist of a sodium hydroxide solution through which chlorinehas been bubbled. This solution may be made by heating a 20% caustlcsoda solution to about 70 C. and then passing chlorine into the solutionuntil it is clouded with liberated oxygen. A 20% solution of sodiumhydroxide at 7 0 to 100 C. is preferred but more concentrated or moredilute alkali solutions may be used. Adilute acid bath consisting of amixture of nitric acid and hydrochloric acid is a suitable acid bath butmore concentrated baths and other pickling acids of variousconcentrations may be used. A suitable pickling bath consists of asolution containing 10 to 100 grams per litre of concentratedhydrochloric acid and about 50 grams per litre of concentrated nitricacid. This bath is used at about 70 C. but other concentrations andmixtures of theae acids and other temperatures may be use The particularcondition of surface oxidation that is suited to my process is believedto be a suboxidized condition. The special scale may be-developed in boxannealing and it is referred to herein as box annealed scale and assuboxidized scale but the precise structure and composition of the scaleis unknown to me. The desired scale may be developed to some extent onall ferrous metals by heating the metals in reducing or non-oxidizingatmospheres, and at times scales amenable to my process are developed inthe usual manufacturing operations without heating in specialatmospheres. The scales on iron and chromium alloys known as tempercolors are readily amenable to my process and are included within themeaning of the terms box annealed scales or suboxidized scales. ome ofthe oxide coatings are almost insoluble in acid and others are solubleto a considerable extent. The more insoluble scales have the property ofbeing bodily loosened or sloughed from the surface of the metal by acidsafter the coatings have been subjected to the alkaline bath referred toherein. This sloughing action may be eflected by acids which do notattack the metal.

Although my process is useful for cleaning all ferrous metals, it isespecially useful for removing acid resistant scales such as thoseoccurring on iron alloys containing 8% or more of chromiuin. Thesealloys contain about 8% to 40% chromium and may contain substantialamounts of other alloying elements such as manganese, tungsten andnickel.

For example, the surface oxide on hot rolled rustless iron may beremoved with acid but when the article is box annealed, the scale ispractically immune to the action of acid. This scale and some of thescales on stainless steel are so difficult to remove with acid thatmethods involving sand blasting and grinding which are so laborious andcostly that the cleaning often amounts to 10% of the manufacturing costof the metal, have heretofore been used. With the processes of myinvention the acid resistant scales on these materials may be removed soas to expose bri ht clean metal with a minimum cost of 1a or andmaterials.

These scales are by no means a product of special processes as theyoccur to some extent on portions of heat treated sheets which are notaccessible to air. WVhere sheets are box annealed under such conditionsthat oxidation proceeds to a considerable extent, the scale on theexposed edges of the sheets are apt to contain only a small amount ofsuboxide constituents or none at all and the unexposed portions aregenerally suboxidized. Such a variation in the scale is undesirable inmy process since thealkali bath acts on the suboxidized scale and givesit its properties of sloughing and quick reaction when it is treatedwith acid. Therefore, a uniform quality of scale of suboxidizedcharacter gives the best results.

When a carefully box annealed scale is treated with the alkali bath andthen with the acid bath, a material which occurs next to the metalappears to be dissolved almost immediately and the main body will sloughoff of itself. However, the rapidity of the sloughing action variesaccording to the composition of the metal and according to thecondiltioning to which the scale has been sub- Jecte Suboxidized or boxannealed scales are not always developed in the ordinary methods ofmanufacture but they occur to some extent, on many metals. For thepurpose of developing the oxide in some instances and of making I auniform coating in other instances, it is desirable with some oxidizedsurfaces and it is necessary with other oxidized surfaces to provide aconditioning. The conditioning may be done for the sole purpose ofchanging the nature of the scale or it may be accomplished in the courseof other methods of manufacture.

The best method of conditioning scale is to subject it to a hightemperature in the presence of a reducing or a non-oxidizing atmosphere.This treatment produces a suboxidized scale of uniform quality on allparts of the article. A uniformly conditioned scale permits of a uniformreaction when the scale is treated with the alkali and subjected to theaction of the acid. It is convenient to condition the oxide during boxannealing. For example, I place the article in an annealing box with afew pieces of charcoal which are out of contact with the metal and thenheat the metal to an annealing temperature, for a short time. Instead ofplacing charcoal in the box, I may displace the air in the box with areducing atmosphere. For chromium alloys such as rustless iron andstainless steel the reducing gas should be free from carbon-containinggases which breakdown and deposit an adherent carbon-containing materialon the metal at these temperatures. 1

As illustrative of my invention, a satisfactory method of cleaning hotrolled sheets of rustless iron containing 12% to 18% chromium, follows.The scale is conditioned by box annealing the sheets at- 600 to 1000 C.in a box in which a small amount of wood charcoal is placed out ofcontact with the metal inorder to produce a neutral or reducingatmosphere. The scale is then treated by immersing the sheets in a 20%sodium hydroxide solution into which chlorine has been bubbled until acloud of liberated oxygen appears. The oxidizing condition in the alkalisolution may be produced by adding a chloride to the bath, electrolyzingthe bath with insoluble anodes to produce chlorine or by other wellknown methods. A treatment of three or four minutes in this solutionatabout 70 C. is usually sufficient but some scales require a longertreatment. Scales that have not been completely conditioned haverequired several hours treatment in the alkaline bath in order to removethem satisfactorily when they were subjected to acid.

When the scale is properly conditioned and treated, it sloughs off in anacid bath as if a layer of oxide next to the metal were dissolved andloosened by the acid. The overlaying layers or admixed portions of scaledrop or peel off. The scale which drops iron-chromium alloys and on themajor por:

tion of the scale is negligible when the acid is dilute. An acid bathcontaining grams per litre of nitric acid and about 10 to 100 grams perlitre of hydrochloric 'acid' works satisfactorily at C. with ironchromium alloys but other concentrations and temperatures may be used.In general, it is preferable to vary the concentration of hydrochloricacid according to the chromium content of the alloy. The more dilutesolutions are used with those alloys which are lower in chromium. A boxannealed scale will ordinarily peel or slough off of itself after anexposure of one or two minutes in the acid bath with- I-ACUHHNI outmaterially attacking the metal or the scale. If the scale does not falloff of itself it may be washed off in water or brushed off.

The acid bath may be and preferably is adjusted so that it will notattack the metal or dissolve the major portion of the scale. It isevident that the use of a bath which does not dissolve the metal or themajor portion of the scale is advantageous since it will be exhaustedless rapidly than a corrosive bath would be and the metal will not bedestroyed. When the scale has been properly condi-. tioned and treatedand an acid bath of this character is used, the metal is left bright,thus obviating the necessity of giving the metal a final brighteningtreatment.

With some scales the alkaline treatment may be advantageously precededby subjecting the scale to the action of acid, but for most purposes theprocess may consist of providing a scale of the sub-oxidized characterwhich may occur as the result of other processes of manufacture or asthe result of a conditioning as hereinbefore described, treat- 7 ing inan alkaline bath which may contain an oxidizing agent and thensubjecting to an acid bath. Since variations of the process other thanthose which have been specifically described may be made withoutdeparting from my invention, I do not wish to be understood as limitingmy invention to the preferre 1 steps or to these modifications.Therefore, I desire to be limited only by the prior art and by theinvention as defined in the annexed claims.

I claim as my invention:

1. The method of removing surface oxides from ferrous articles whichcomprises subjecting the same to an alkaline bath containing anoxidizing agent and to an acid bath.

2. The method of removing surface oxides from ferrous articles whichcomprises subjecting' the same to a chlorinated alkaline bath and to anacid bath.

3. The method of cleaning a surface oxidized article containing ironwhich comprises providing a box annealed scale thereon. subjecting thearticle to an oxidizing alkaline bath containing chlorine and then to anacid bath.

4. The method of removing surface oxides from articles containing ironand at least 8% of chromium which comprises subjecting the same to analkaline bath containing an oxidizing agent and to an acid bath.

5. The method of removing surface oxides from articles containing ironand at least 8% of chromium which comprises subjecting the same to achlorinated alkaline bath and to an acid bath.

box annealed scale thereon, subjecting the ar-- ticle toa chlorinatedoxidizing alkaline bath and to an acid bath containing nitric andhydrochloric acids.

8. The method of cleaning a surface-oxidized article containing iron andat least 8% of chromium which comprises providing a box annealed scalethereon, treating the article to a hot bath containing about 20% ofsodium hydroxide in which chlorine has been bubbled until oxygen isliberated and subjecting to a hot acid bath containing about 50 gramsper liter of nitric acid and 10 to 100 grams per liter of hydrochloricacid.

9. The method of cleaning a surface-oxidized article containing asubstantial amount of iron which comprises heating in a closed box in anatmosphere which has access to a reducing agent, treating in an alkalinesolution containing an oxidizing agent and then subjecting to an acidbath.

10. The method of cleaning a surface oxidized article containing asubstantial amount of iron which comprises heating in a closed box in anatmosphere which has access to wood charcoal, treating in an alkalinesolution containing an oxidizing agent and then subjecting to an acidbath.

11. The method of cleaning an oxide coated article containing asubstantial amount of iron which comprises heating in'an atmosphereunder conditions which will produce partial reduction of the oxidematerial, treating in an alkaline bath containing an oxidizing agent andthen subjecting to a dilute acid bath so as to slough the oxide coatingfrom the article.

12. The method of removing surface oxides 4 from ferrous articles whichcomprises provlding a box annealed scale thereon, sub ectmg the same toan alkaline bath containing an oxidizing agent and to an acid bath inwhich the metal of the article is not materially soluble.

13. The method of removing oxide from ferrous metal articles consistingin first immersing the articles in a hot bath consisting of a solutionof caustic soda and an oxidizing agent to break down the resistance ofthe oxide to acid, and then immersing the articles so treated in a hotacid pickling bath.

14. The method of removing oxide from stainless steel articlesconsisting in first immersing the articles in a hot bath consisting of asolution of caustic soda and an oxidizing agent to break down theresistance of the oxide to acid, and then immersing the articles sotreated in a hot acid pickling bath.

In testimony whereof, I aflix my signature.

HARRY S. GEORGE.

